1. Field of the Invention
This invention relates to a vehicle behavior control system, more particularly to a vehicle behavior control system to control vehicle behavior so as to overcome an unintentional steering wheel movement caused by a force generated by a tire/road interplay (interface friction) due to a sudden automatic braking produced under obstacle avoidance control.
2. Description of the Related Art
There have hitherto been proposed obstacle avoidance controls. For example, Japanese Laid-open Patent Application Hei 6(1994)-298022 teaches detecting an obstacle (e.g., another vehicle running ahead on the road) with the use of an obstacle detector such as a laser radar and automatically operating the braking system (i.e., independently of the vehicle operator's brake pedal manipulation) so as to avoid contact with the obstacle.
In the prior art technique, specifically, the acceleration of the other vehicle (obstacle) is detected and, based on the detected acceleration, a first threshold value (relative distance) for avoiding the obstacle by braking is determined. At the same time, a second threshold value (relative distance) for avoiding the obstacle by steering is determined based on the assumption that the subject vehicle moves to avoid the another vehicle with a lateral acceleration b0 after time .tau., measured from a certain point.
In such an obstacle avoidance control, the braking system is operated automatically to immediately produce large braking forces when the possibility of contact with the obstacle is found to be great. When the vehicle is running on rough terrain where the coefficient of tire/road interface friction is relatively great, a force generated by the tire/road interplay (interface friction) due to the produced sharp braking forces may sometimes cause the steering wheel to be moved other than as intended by the vehicle operator, thereby causing the vehicle to deviate from the course of vehicle travel intended by the vehicle operator.
Similar problems may arise from the large automatic braking forces when the vehicle is running on a road where the coefficients of tire/road interface friction acting on the left and right wheels are different from each other.
When this happens, the tires are subjected to side force which lowers the gripping force in the vehicle longitudinal direction. As a result, the vehicle driving behavior is degraded, which may sometimes cause the vehicle to deviate from the desired course of travel as mentioned above. This also causes other problems to arises. For example, the vehicle may fail to attain the desired deceleration or it may fail to achieve the desired obstacle detection necessary for an effective obstacle avoidance control.
Aside from the above, it has been known to equip a power-assisted steering actuator in the steering system such that the steering force produced by the muscular energy of the vehicle operator is assisted by the steering actuator.